1. Technical Field
This invention relates to an optical recording medium whose label surface is printable or writable with an ink containing a water-soluble dye.
2. Background Art
Optical recording media, especially optical recording media like recordable compact disks (CD-R) are required to write or print record contents on their surface where media's label is conventionally printed, to be referred to as a label surface, hereinafter. There are some media whose label surface is toughened so as to be writable with a pencil, but such writing can damage the optical recording layer to increase errors. Then such optical recording media favor recording by an ink jet recording system capable of printing without contact with a surface layer to be recorded.
The ink used in the ink jet recording system generally contains water-soluble dyes such as acidic dyes and direct dyes and water solvent as main ingredients and in most cases, further contains small amounts of polyhydric alcohol.
In the prior art, the substrates to be recorded for use in the ink jet recording system are plain paper and substrates having a hydrophilic surface, typically ink jet dedicated special paper having an ink receptive layer. Also many substrates are provided on the surface with a printable layer which is a water-soluble polymer layer formed of starch, gelatin, casein, gum arabic, sodium alginate, carboxymethyl cellulosem polyvinyl alcohol, polyvinyl pyrrolidone, and sodium pclyacrylate, or a layer of a synthetic resin latex such as synthetic rubber latex and an organic solvent-soluble resin such as polyvinyl butyral and polyvinyl chloride. Another proposal is to use polymers having canions. This proposal is intended to prevent redissolving of ink by ionic bonds because many water-soluble dyes for ink jet printing have anions such as sulfonic acid. Known such cationic polymers include, for example, polyvinyl alcohol, polyacrylate and polyacrylamide having a quaternary ammonium salt group (see JP-A 228984/1986, etc.). Use of polyethylene glycol dicarboxylic acid is also proposed for the purpose of improving color (see JP-B 1706/1992).
These water-soluble polymers are highly hydrophilic and thus well compatible with substrates if the substrates used are hydrophilic ones, typically paper. Then no problems arise with respect to adhesion or the like when a printable or writable layer is provided on the substrate surface using such polymers. However, since the water-soluble polymers lack adhesion to substrates having a highly hydrophobic surface as in the case of ultraviolet-cured resins frequently used as protective films for optical recording media, a layer, once applied thereon, will readily strip off or a printable layer itself is less resistant to water so that after printing, ink will be seriously oozed with water droplets, leaving problems on practical use.
If films of ultraviolet-cured resin as used in prior art optical recording media are provided in order to provide substrates having a hydrophobic surface with a printable layer which is relatively free from problems of stripping and water resistance, the films are repellent to ink containing a water-soluble dye and thus unprintable.
Also proposed is a laminated structure wherein a layer which is well adhesive not only to a hydrophobic surface, but to a hydrophilic polymer layer is formed on a substrate having a hydrophobic surface and a hydrophilic polymer layer is formed thereon as a printable layer. This increases the number of steps, resulting in an increased cost.
On the other hand, printable substrates which are used in the ink jet recording system include plain paper and ink jet dedicated special paper which is provided with a porous ink-receptive layer in order that polyhydric alcohol and other less-volatile components in the ink be absorbed and dispersed in the ink-receptive layer to accelrate ink drying and fixing.
Where substrates are hydrophilic and porous as in the case of paper, the substrates by themselves have the function of an ink-receptive layer. However, where substrates are non-porous, a long time is taken for drying and fixing due to less-volatile components in the ink. If any object comes in contact with a printed area before drying, the ink transfers to the object or prints are impaired. Then such substrates are conventionally provided with an ink-receptive layer containing porous particles. The known porous particles used in the ink-receptive layer include inorganic pigment particles such as silica, clay, talc, diatomaceous earth, calcium carbonate, calcium sulfate, barium sulfate, aluminum silicate, synthetic zeolite, alumina, zinc oxide, lithopone, and titanium white (JP-A 228984/1986, JP-B 1706/1992, etc.).
As mentioned above, the substrate surface on the label surface side is typically a protective film of ultraviolet-cured resin which is hydrophobic and has a non-porous surface. An attempt to directly write or print on the label side surface by the ink jet recording system fails because the protective film is repellent to the ink.
Then in order to enable printing by the ink jet recording system, we attempted to provide an ink-receptive lawyer containing porous particles on the protective film. This attempt, however, failed to achieve satisfactory results as printable members because of a low drying rate of ink after printing and poor water resistance of prints.